In order to adhere to increasingly stringent emission thresholds it is necessary to detect leaks reliably in a tank vent system of a motor vehicle. These checks prevent fuel vapors from escaping unnoticed into the environment.
A method for detecting a leak in a tank vent system is known from the patent specification U.S. Pat. No. 5,263,462. The method makes use of the natural formation of a vacuum inside the tank vent system. Once the tank vent system has cooled down sufficiently after the motor vehicle has been switched off, a test is conducted as to whether or not a pressure switch arranged in the tank vent system is closed. The closing of the pressure switch indicates a decrease in the pressure within the tank vent system to below a pressure value. In the case of a closed pressure switch, a leak within the tank vent system can be ruled out, because a natural vacuum could form inside the tank through cooling. On the other hand, an open pressure switch is regarded as a sign of a leak within the tank vent system. This method requires an operable pressure however. However, in the case of a defective pressure switch an incorrect diagnosis can be made and therefore testing the operability of the pressure switch is required.